The present invention relates to an infrared gas analyzer for qualitatively or quantitatively analyzing a component gas to be analyzed and contained in a gas to be measured by detecting an amount of infrared rays absorbed.
An infrared gas analyzer irradiates a gas to be measured with infrared rays to measure an amount of infrared rays with a particular wavelength region absorbed by a component gas to be analyzed and contained in the gas to be measured in order to qualitatively or quantitatively analyze the components of the gas to be measured based on the amount of infrared rays absorbed. This method is commonly used because it generally offers good selectivity and high measurement sensitivity.
FIG. 10 is a cross sectional view showing a structure of an example of a conventional single-beam infrared gas analyzer. A source of infrared rays 3 has an emitter such as a nichrome wire, and the emitter is supplied with electricity for heating to emit infrared rays. The infrared flux emitted from the infrared source 3 is changed into discontinuous or intermittent light by a rotating chopper 2 driven by a motor 1 and is supplied to a measuring cell 5 as a measured beam 4. The measuring cell 5 is formed like a cylinder with infrared transmission windows 51 and 52 attached at both ends, and the beam is inputted and outputted through the windows. In addition, the measuring cell 5 includes an inlet tube 53 and a discharge tube 54 for introducing and discharging a gas to be measured. In the measuring cell 5, the measuring beam 4 is subjected to absorption depending on the concentration of a component gas in the gas to be measured and introduced into the measuring cell 5. The measuring beam 4 passing through the measuring cell 5 is ejected to a detector 6 that detects infrared rays of a particular wavelength band.
The detector 6 has a first and second expansion chambers 63 and 64 in series with the direction in which the measuring beam 4 advances, so that the measuring beam 4 passing through the measuring cell 5 enters through an infrared transmission window 61 the first expansion chamber 63 which is the front chamber. The measuring beam 4 passing through the first expansion window 63 further passes through an infrared transmission window 62 to the second expansion chamber 64 that is a rear chamber. The same gas as the component gas to be analyzed is sealed in both expansion chambers 63 and 64. Infrared rays with wavelength components having a high absorption coefficient in terms of the wavelength-infrared rays absorption characteristic of the sealed gas are mostly absorbed in the first expansion chamber 63, which is the front chamber, while the wavelength components that have not been absorbed in the first expansion chamber, that is, those having medium absorption coefficients, are mainly absorbed in the second expansion chamber 64, which is the rear chamber.
A temperature of the sealed gas increases depending on the amount of infrared rays absorbed, to thereby increase the pressures of the expansion chambers 63 and 64. The sealed gas flows in a gas passage 65 based on the pressure differential between the expansion chambers 63 and 64 which is caused by the difference in the amount of infrared rays absorbed, that is, the concentration of the analyze-component gas in the gas to be measured. The concentration of the analyze-component gas in the gas to be measured can be detected by measuring the flow rate.
FIG. 11(a) is a plan view of heating coil elements 66 and 67 for detecting the gas flow rate. FIG. 11(b) is a cross sectional view taken along line 11(b)--11(b) in FIG. 11(a). FIG. 11(c) is a circuit diagram of a detection circuit.
The heating coil elements 66 and 67 are made of an electric conductor in which the resistance value sensitively varies with a temperature, that is, one having a high resistance temperature coefficient (for example, nickel). In FIGS. 11(a)-11(c), a rectangular foil of nickel is etched from both sides so as to be formed into a zigzag resistor as shown in FIG. 11(a), and two such resistors are disposed adjacent to each other and integrally fixed to each other at their outer circumferences as shown in FIG. 11(b). The center areas of the resistors form an opening through which a gas can flow. Such a pair of heating coil elements 66 and 67 is disposed in the gas passage 65 in the longitudinal direction relative to the gas flow.
The pair of heating coil elements 66 and 67 is combined with a pair of fixed resistors 68 and 69 so as to constitute a Wheatstone bridge circuit as shown in FIG. 11(c), and is heated by a current provided by a power supply connected to the Wheatstone bridge to increase a temperature above the ambient temperature. In addition, due to their relative proximity, the heating coil elements 66 and 67 thermally affect each other.
When there is no difference in pressure between the first and second expansion chambers 63 and 64, since the gas does not flow, temperatures of the two heating coil elements are balanced with the ambient temperature. If there is a difference in pressure between the first and second expansion chambers 63 and 64 and the gas flows through the gas passage 65, a temperature of the heating coil element on the upstream side of the gas flow is decreased by direct contact with the gas flow, while the downstream heating coil element contacts the gas flow heated by the upstream heating coil element and thus becomes hotter than the upstream element. In this manner, the temperatures of the heating coil elements 66 and 67 vary with the intensity of the gas flow through the gas passage 65, that is, the difference in pressure between the first and second expansion chambers 63 and 64. This variation is detected by the output from the Wheatstone bridge circuit. Since this output is in proportion to the concentration of the analyze-component gas contained in the gas to be measured, the concentration of the analyze-component gas can be measured by using the output from the Wheatstone bridge circuit.
If the infrared gas analyzer of the structure shown in FIG. 10 is used to detect a low density analyze-component gas contained in the gas to be measured, since a smaller amount of infrared rays is absorbed, a sufficient output can not be obtained. The required output has thus been obtained by increasing the length of the measuring cell 5 and/or the radiant intensity of infrared rays from the infrared source. This method, however, has the following problems.
First, the content volume of the measuring cell 5 increases along with the increase of the length of the cell. When the content volume increases, an amount of gas to be measured required for analysis increases to prevent trace levels of gas from being analyzed; time required to substitute the gas to be measured in the measuring cell 5 increases, to thereby delay the response; required capacities of a pump apparatus and preprocessor for removing dust and moisture in the gas to be measured increase, leading to greater physical sizes and costs for these devices; and the resulting increase in size of such an apparatus leads to an increase in the thermal capacity, thus increasing the time required for warming up the apparatus. In addition to these problems, a more serious problem is that if the analyzer is used in an environment involving petroleum or petrochemical processes, which require an explosion-proof structure, the size, weight and cost of the equipment become huge. Since the infrared source used in this type of the infrared gas analyzer is also a source of heat, the light source must at least be housed in a pressurized container in order to be used in an explosion-proof environment as described above. Since, however, optically transparent windows required to obtain infrared rays have a low pressure resistance, the entire infrared gas analyzer body including the measuring cell and the infrared detector must be housed in a pressurized container, so that the reduction of the size of the components such as the measuring cell and the infrared detector is an important practical matter.
Second, an increase in the radiant intensity of infrared rays from the infrared source increases the calorific value of the infrared source, so that time required to stabilize its temperature increases, thereby increasing the time required for warmup operations.
Third, the conventional infrared gas analyzer has an outside-air-flowing layer in an optical path extending from the infrared source to the infrared detector. When a low density component gas to be analyzed is detected and if the component gas having an absorption band in the same wavelength region as in the component gas to be analyzed is present in air and its concentration varies, then not only the measured beam 4 is absorbed in the measuring cell 5 but the infrared rays in the absorption band of the analyzed-component gas are also absorbed in that part of the optical path extending from the infrared source to the infrared detector, which passes through the outside layer. Thus, the measurement sensitivity decreases to degrade the measurement accuracy, preventing low concentrations from being detected easily. One example is the effect of carbon dioxide (Co.sub.2) in air in the measurement of the concentration of carbon monoxide (CO).
A multi-reflection measuring cell originated by White (hereafter referred to as a "White cell") has been used to solve the first of the above problems. This cell has a structure such as that shown in FIG. 12. In this structure, a condensed beam of infrared rays is applied on a multi-reflection optical system and travels or passes back and forth within a small space to establish a long optical path. It is described in the following documents and is publicly known.
(1) J. U. White, J. Opt. Soc. Am., vol. 32, 285 (1942) PA1 (2) J. U. White, N. L. Alpert, A. D. DeBell, J. Opt. Soc. Am., vol. 45, 154 (1955) PA1 (3) P. Hannan, Opt. Engineering, vol. 28, 1180 (1989)
The structure of the White cell 7 is described in detail with reference to FIG. 12.
A central concave mirror 75, an input image-forming mirror 76, and an output image-forming mirror 77 are concave mirrors with the same radius of curvature and disposed in such a way that the distance between opposite reflecting surfaces is equal to the radius of curvature. An incident window 71 used to introduce an incident beam 41 is provided adjacent to the central concave mirror 75 as a slit or a small hole, while an input image-forming mirror 76 is disposed opposite to the incident window 71 such that it forms on the central concave mirror 75 an image of the incident beam 41 through the incident window 71. The beam reflected from the central concave mirror 75 is reflected by the output image-forming mirror 77 disposed next to the input image-forming mirror 76 and again forms an image on the central concave mirror 75. After such reflective image forming is repeated for the required number of times, the beam reflected by the output image-forming mirror 77 is emitted through an emitting window 72 disposed adjacent to the central concave mirror 75 and opposite to the incident mirror 71. The incident beam 41 emitted into the White cell 7 is a condensed beam that enables a reflected image to be formed many times within the White cell 7.
In FIG. 12, the incident beam 41 is reflected by each of the input and output image-forming mirrors 76 and 77 four times and by the central concave mirror 75 seven times in the alphabetical order of (a) to (g), and travels back and forth eight times between the concave mirrors before being emitted as an emitted beam 42.
Thus, due to its ability to substantially reduce the content volume of the measuring cell, the White cell 7 is very effective means for solving the first problem. The White cell, however, does not serve to sufficiently reduce the size of the conventional infrared gas analyzer that is a simple combination of the infrared source 3, the White cell 7 as the measuring cell, and the infrared detector, and therefore requires a relatively large area in between these elements. The reason for this is described below.
Optical components including several concave mirrors are used for the White cell 7 and a condensing optical system in the preceding stage, and must be disposed and oriented very accurately to achieve the required accuracy of their optical axes. This accuracy can not be maintained easily due to the thermal expansion of the members caused by radiant heat from the infrared source 3, and as a result, a predetermined amount of light or a required optical path length may not be obtained. Consequently, the analyzer must have a structure such that the optical system can be adjusted. That is, it is difficult to integrally form the analyzer. Although an example of integration of the measuring cell and the infrared detector has been shown in a catalog from DASIBI, no conventional analyzers are integrally formed up to the periphery of the infrared source.
With respect to the second problem, the infrared source 3 must provide a certain amount of light to obtain the required output level. While this requirement is met, the size of the analyzer must be reduced. Of course, the size of the infrared source 3 is desirably minimized to obtain good results including increased use efficiency, but to do this, a temperature of the emitting section of the source must be increased. In this case, the radiant heat also increases to cause an increase in temperature and thermal expansion, both of which are more serious problems.
Solutions to the third problem are described below.
One method is to form the entire infrared gas analyzer such that a space in the optical path extending from the infrared source 3 to the infrared detector 6 through which the outside air can pass is minimized, while keeping the structure of the infrared gas analyzer unchanged.
A second method is to seal the entire infrared gas analyzer in an atmosphere filled with a gas that does not absorb infrared rays, for example, a nitrogen gas.
A third method is to increase the substantial length of the measuring cell up to a value at which the effects of the outside air portion are negligible.
However, the first method is insufficient, and the second method results in a complicated configuration and thus an increased size due to the addition of a structure that must enclose the entire analyzer. The third method is inconsistent with the reduction of the size of the device.
It is an object of this invention to solve the above problems of the conventional infrared gas analyzers and to provide a small infrared gas analyzer that can detect and analyze a low density component gas to be analyzed and that can provide an explosion-proof environment.